An antenna module of this type integrates various functions of vehicle roof antennae. There are known microstrip-patch-antennae, that include a substrate, which in turn features metallization of the entire area of its lower surface and a suitable metallic structure or antenna structure on its upper surface. Antennae of this type commonly have a very narrow frequency bandwidth, for example, 1% to 2% relative bandwidth, unless additional measures are taken. By employing parasitic elements, bandwidth can be increased or multiple frequency bands can be blocked. These parasitic elements are conduction or surface structures, which are present on the same or higher plane than that of the antenna structure. If the parasitic elements are on a higher antenna structure, then they are coupled to the lower antenna structure, wherein a common HF-connection cable on the lower antenna structure runs to an amplification unit. In the parasitic elements, high frequency currents are induced, which adapt to the shape and dimensions of the parasitic elements and thereby produce fields. As a result, the entire structure has the capacity to send and receive both neighboring as well as somewhat distantly spaced frequencies.
Antenna structures of this type are only suitable, if the entirely expanded frequency band is allocated for the same service.
As a rule, if multiple, independent services are intended, then antenna modules with separately built antenna elements arranged next to one another are then used. In this configuration, however, more space is required. Furthermore, for the proper function of the individual antennae elements, sufficient isolation is required.
EP 0 521 384 A1 describes an antenna module with an upper and lower substrate, whereby an upper λ/2-antenna structure is present on the upper substrate and a lower λ/2-antenna structure is present on the lower substrate. In both antenna structures a metal layer present beneath the lower substrate serves as reference plane, facilitating a parallel connection of the oscillating circuits of both antennae.